This invention relates to earth grading and paving machines, and specifically relates to a method for automatically controlling the depth of the blade carried by such a machine for maintaining a desired grading or paving height relative to a reference surface.
In conventional grading vehicles, an operator of the vehicle will set the height of the blade on a surface to be graded at a particular depth relative to a reference surface. Typically, the operator may grade a portion of the gradable surface until the proper depth is reached, and then will continue with the blade at that height for the remainder of the job. The reference surface may be an adjacent portion of ground, a standard string line against which vertical measurements may be taken, or a roadside curb. In the contact method of following, the string line or other reference surface is actually contacted by the follower to detect changes in the height of the reference surface.
A problem with the conventional approach is that the reference surface will typically vary in height, often by a very significant amount. However, the graded surface is usually required to be within closed tolerances. Thus, a need exists for a reliable means of maintaining the blade at a constant depth relative to the reference surface, despite variations in the height of the latter.
In some prior art systems, such as in the Davidson et al. U.S. Pat. No. 4,733,355, an acoustic sensor has been used to follow the reference surface. Such prior art systems, however, have several drawbacks. One is that the acoustic sensor must be manually calibrated by the operator of the vehicle, who must measure the height of the sensor relative to the reference surface and/or the depth of the blade, and must physically adjust the blade height (using a ruler to measure the height) in order to calibrate it for a chosen grading depth. Normally, this is done by a two-man operation. The prior art system uses an acoustic signal echo to measure distance, and uses timing windows to determine the approximate round trip time of the return echo. A green light and an "high" and "low" signals are used to signal to the operator when the blade is on the proper grade, is too high or is too low, respectively. There is an acoustic generator unit which is mounted on the side of an earth mover and which generates an acoustic signal which is directed toward the reference surface.
To use the prior system, the operator of the earth mover must set the blade at the proper height relative to the reference surface. This requires finding a place where the grade and the reference surface are already separated by the desired amount and setting the blade there, or digging the appropriate grade. In the latter case, an assistant must follow behind the grader or paver with a measuring stick to measure the grade for the driver of the earth mover, so as to indicate when the appropriate grade has been achieved. If an assistant is not available, then the operator of the vehicle must repeatedly adjust the blade height and then climb down off the grader and measure the distance between the reference surface and the graded surface. If the separation of these two surfaces is not correct, the above-described process must be repeated until the desired offset is achieved. Once the correct grade is arrived at, the acoustic transmitter must be manually adjusted until a green light indicating a "lock-on" at the appropriate height is generated. This process is cumbersome and time consuming. If two workers are used, there are considerable extra expenses and availability problems. Accordingly, a need has arisen for a system which can automatically "lock on" to the correct blade height with only one worker.
Another drawback of present systems is that they make no provision for the fact that a grader or paver may be used over a period of many hours during the day, during which time the temperature may change significantly. As air temperature in the vicinity of the sensor changes, errors are introduced into the following ability of the system, because changes in temperature cause changes in air density, which in turn lead to changes in the speed of sound. Since the sensor results depend on travel time of the pulse, errors result when temperature changes are not compensated. Moreover, in the prior systems, adjustments for such errors would have to be made manually.
Another drawback of systems presently in use is that they provide no indication to the operator of the grader when the reference surface height varies outside the range of sensitivity of the system. Thus, the height of the reference surface may rise or drop drastically, and for such changes it is undesirable to make concomitant drastic changes in the depth of the graded surface. Prior art systems do not provide indications to the operator of when the height of the reference surface exceeds the acceptable limits.